The lineal seam welded steel pipe is produced mainly as the so-called seam welded steel pipe, which is formed by a continuous roll forming of the band steel by using the forming roll suitable for forming the required diameter and thickness and arranged according to the requirement as to its diameter, which is most suitable for the mass production concentrated in a fewer kinds of items.
In the production of steel pipes by the continuous roll forming, the whole process is roughly divided, as is shown in FIG. 15, into the edge forming section I of the band steel, the center forming section II and the reducing section III, in which the initial forming from the edge bending to the half-circularizing are effected by the horizontal roll clusters (FIG. 15-b), then formed substantially to the circle by non-driven side roll clusters (FIG. 15-c), and the angle adjustment of the edge, reducing, finishing and centering are performed by so-called fin pass rolls (FIG. 15-d), then led to the welding process IV by squeeze rolls.
In the above process, the horizontal and side rolls are usually arranged in many combinations, and furthermore for the purpose of preventing the edge stretch occurring when forming the relatively thin pipe, a cage forming process arranged with a number of small rolls having a flat surface called cage rolls is used together.
In such conventional continuous roll forming process, the initial forming stage is important for mainly forming a half-circle in the initial stage, is spite of the fact that the edge bending of the band steel is most difficult and generally influencing the proper roundness of the pipe and the quality of welding process is insufficient and numerous cases of springing back are produced after the forming due to the poor forming, mainly from the requirement of production cost, the method of forming of center portion in the following forming region is performed by adding the forming stages without completely finishing the edge bending, and the whole process is thus chosen to compensate such insufficient forming in the initial and intermediate stages in a breath in the reducing region.
Accordingly, since the additional forming in the reducing region due to the insufficient forming in the initial and intermediate stages is not desirable from the point of proper distribution of forming processes in the whole process when the accuracy and productivity of the product and economical aspects of the forming apparatus is considered, besides that insufficient edge bending and the so-called angular distortion in the intermediate forming stage will influence the proper roundness of the pipe and the welding quality, the forming process or the forming apparatus which performs the edge bending in the edge bending stage sufficiently, or compensates the insufficient bending in the following stage to prevent the angular distortion was an obvious need.
Furthermore, as previously described, the production of welded steel pipe by the continuous roll forming method requires the forming rolls suitable for forming to the required diameter and thickness, properly arranged in accordance with its required width of the material to be formed, thus a considerable number of forming rolls should have to be prepared for every required diameters. This is not desirable from the production efficiency, and consequently this process has been used for the mass production of smaller items. Recently, therefor, the efficient production of large items with a single machine is needed, that is, the continuous roll forming process or the forming apparatus suitable for the small quantity production of large items, not to speak of mass production of small items, is demanded.
Moreover, in the double radius forming, because the edge bending curvature will change as the diameter varies, when a common roll is used with the bending portion not possible to be pressed and the circular portion being pressed as contacting with straight rolls or rolls having relatively larger radius of curvature at the points, thus the formability of roundness is deteriorated and the angular distortion tends to occur.
When a plurality of abovementioned cage rolls are utilized to use the roll commonly, similarly a flat roll should be used, resulting in the material being formed tending to roll.
Meanwhile, the forming roll having three kinds of curved surfaces is proposed in Japanese Patent No. 46926/82 as the break down forming roll for several kinds of pipes having different diameters.
More specifically, the abovementioned break down forming roll includes the roll surface, in which the curved surface having the center angle of 15.degree.-45.degree. and the bending radius required for bending the edge of the band steel of the pipe having a maximum planned diameter, the curved surface having the center angle of 40.degree.-55.degree. and the bending radius required for bending the edge of the band steel of the pipe having a minimum diameter, and the curved surface having the center angle of 5.degree.-45.degree. and the bending radius required for the break down of the inner curved surface following the edge portion of the band steel of said minimum diameter pipe, are arranged in sequence from the outside to inside of the roll surface, the break down rolls being respectively arranged in the multi-stage stand so as to complete the break down of the forming section 1 of the edge portion of the band steel mentioned above.
In the break down roll, since it is intended to use the roll commonly within the several pipe diameter ranges, and to decide three kinds of curved surfaces of the roll on the basis of maximum and minimum pipe diameter in the commonly used range, in the edge bending of the band steel performed at the required multi-stage conventionally used, similarly as in the past, the multi-forming is essential and the number of forming stands could not be reduced, besides the distribution of forming is exactly as same as the convention process mentioned above, so that the insufficient formability occurred in the initial and intermediate stages is planned to be corrected at a breath in the reducing region.